Frequent Disconnects - "ppp0 LCP down"

We've been plagued with random disconnects for several months now (long enough ago that I can't remember if we had the troubles before we switched routers). It affects me in the evening, but what I didn't realise is my partner has has these issues during the day while I'm out, and they've had to reboot the router at least once daily to solve it.

I've got a PC that is wired up to our router. The router also has WiFi capabilities, which are used with several other devices (laptops, phones etc.)

Every so often (hourly, roughly, but can be sporadic) we sort-of lose our internet connection. On the wired PC, if I'm in a discord channel, the audio will cut but will not tell me I've disconnected (if I reconnect, it works fine). If I'm playing a game, it will freeze and eventually disconnect. For games which support reconnecting, it will hang until it times out or I close the game. The weird thing about it is that if I open a browser up I can still browse. The only game I've managed to get running & keep connected is a one called "7 Days to Die", because it has an option to "Disable UNET Networking" Which seems to be some sort of Unity thing.

For wireless devices, any kind of media streaming will stop & it takes some kicking to get back started again. More often than not the problem is temporarily resolved after a router reboot. The devices have full signal strength to the router, but can't seem to use the internet.

The router is connected directly to the BT OpenReach modem, then straight into the wall. We're on the unlimited plus package. We should also be using a static IP.